1. Field of the Invention
The present invention relates to a Fiber To The Home (FTTH) system based on a passive optical network (PON) capable of providing subscribers with one or more of a high-speed data service, a voice service, and a broadcasting service using an optical fiber instead of an existing telephone line. More particularly, the present invention is related to a structure of a PON-based FTTH system and a method for establishing the same to provide a satellite broadcasting service by utilizing a conventional overlay CATV broadcasting service system in order to match the various/discriminated requirements of subscribers.
2. Description of the Related Art
With regard to the construction of access networks from a telephone office to a building and a home, there have been some recently developed network structures and methods for improving the various networks. Some examples of the various network structures and the methods for improving same include the use of an x-digital subscriber line (xDSL), a hybrid fiber coax (HFC), a fiber to the building (FTTB), a fiber to the curb (FTTC), a fiber to the home (FTTH) and so forth. Among such structures and methods, the FTTx (x=B, C, H) can be classified into both an active FTTx embodied as a structure of an active optical network (AON) and a passive FTTx embodied as a structure of a passive optical network (PON).
At present, since a PON (when considered in relation to a passive FTTx) has a point-to-multipoint topology using passive elements, the PON has been suggested as a possible access network of the future because of its economical efficiency. In other words, a PON connects an optical line terminal (OLT) to a plurality of optical network terminals (ONTs) through a single optical fiber by means of a 1×N passive optical distribution network (1×N passive ODN), thereby forming a distribution topology of a tree structure.
FIG. 1 is a schematic view showing an entire structure of a conventional PON-based FTTH system. The PON-based FTTH system provides a broadcasting and communications convergence type access network that is capable of providing subscribers with high-speed data services, voice services, and broadcasting services. Referring to FIG. 1, in the PON-based FTTH system, a CATV Tx 111, which is positioned at an optical line terminal block 110, receives CATV broadcasting signals input from a broadcasting service network (HFC network) 107 and outputs an optical signal having a predetermined wavelength of λ1. An OLT 112 generally includes a down stream optical transmitter and an upstream data optical receiver and makes data communication with a data service network 108, such as high-speed Ethernet, etc. The CATV optical signals output from the CATV Tx 111 are then combined with optical signals of data from the OLT 112 by a wavelength multiplexing division-coupler (WDM-coupler) 113. Thereafter, the combined optical signals that are output by the WDM-coupler 113 are then input to an optical distribution network (ODN) 104. Herein, the ODN 104 is an optical transmission medium connecting the OLT block 110 to an ONT block 120. Such an ODN 104 has a variety of arrangements that depend in part on the various network structures. However, the ODN normally includes an optical power splitter, which is a passive component, and a single mode fiber (SMF). After the combined optical signals of the OLT 112 and the CATV Tx 111 passthrough the ODN 104, such signals are then input to the ONT block 120, whereby the input optical signals are split by a WDM-coupler 123 so as to be input to a CATV Rx 126 and an ONT 125, respectively.
As described above, a CATV broadcasting service is provided through an overlay method in the PON-based FTTH system, and this service is easily achieved by applying a CATV broadcasting optical transmitter and a CATV broadcasting optical receiver, which are installed between the H/E (Headend) of the HFC network 107 and an optical network unit, to a PON system.
In the current state, the CATV broadcasting optical transmitter may use a wavelength band of 1310 nm depending on the network structure of the HFC network. However, there is a great deal of difficulty in using the wavelength band of 1310 nm when constructing a network using an FTTH access network system. One reason for the difficulty is that even though the wavelength b and of 1310 nm is subject to less dispersion than a wavelength band of, for example, 1550 nm, the wavelength band of 1310 nm represents a greater loss than the wavelength band of 1550 nm. In addition, optical fiber amplifiers that are currently in use may not be adaptable for the wavelength band of 1310 nm. Accordingly, if the optical fiber amplifiers are not adaptable for 1310 nm, it becomes very difficult to increase the number of subscribers if a network is constructed by using a PON-based FTTH system.
On the other hand, an optical transmitter of the wavelength band of 1550 nm, which may be partially used depending on a structure of the HFC network in the HFC network, mainly employs an external modulation method for use in long distance transmissions from H/E to ONT. However, the optical transmitter that employ the external modulation method require higher operational costs when compared with optical transmitters that employ a direct modulation method, so that costs for a broadcasting service may increase.
Also, current FTTH systems have structures for providing only CATV broadcasting services input through the HFC network. In other words, the currently used FTTH systems have no alternative/backup plan for subscribers' requirements for satellite broadcasting services.